1. Field of the Invention
The present invention relates generally to improvements in track following methods in disc drives, and, more particularly, but not by way of limitation to improvements in thermal calibration of offsets of a servo transducer from servo tracks on a dedicated servo surface to align data transducers with data tracks on data surfaces on which computer files are stored.
2. Brief Description of the Prior Art
In hard disc drives having a plurality of stacked, rotating discs for storage and retrieval of computer files, the files are stored along concentric data tracks on the surfaces of the discs using data transducers proximate the disc surfaces to write the files and, subsequently, to read them. In order that a file, once written, can be retrieved, records are maintained of the tracks and disc surfaces to which the file is written and the disc drive is comprised of a transducer select logic circuit that selects one of the data transducers for writing the file and a servo system that is used to radially position the data transducers, as a group, for selection of data tracks on the surface that is to receive the file. Thus, once a file has been stored, it can be retrieved by moving the transducers to the selected track and reading the file using the transducer that was used to write it.
To control the radial position of the transducers, the transducers are mounted on an electromechanical actuator and servo patterns, written to at least one of the disc surfaces, are sampled by the servo system to obtain transducer position information from which actuator correction signals can be generated and transmitted to the actuator to reposition the actuator and thereby maintain radial alignment between the transducers and tracks to which a file is to be written or from which a file is to be read while the servo patterns may be embedded in the files, a more common approach in disc drives that include a plurality of discs is to use one disc surface as a dedicated servo surface, to which the servo patterns are written during manufacture of the disc drives, and use the mounting of the transducers on a common actuator to position the data transducers by positioning the servo transducer, the transducer proximate the dedicated servo surface, to follow servo tracks defined by the servo pattern on the servo surface. To this end, the data tracks on separate disc surfaces are organized into cylinders, each of which includes a servo track on the servo surface, and control of the radial positions of the transducers is effected on a cylinder basis. Since, only one disc surface need contain servo patterns in the case in which a dedicated servo surface is used, a net gain in disc drive storage capacity can often be achieved when the disc drive is comprised of a plurality of discs. Specifically, the elimination of servo patterns on the remaining surfaces; that is, the data surfaces, more than compensates for the loss of availability of the dedicated servo surface for the storage of files.
While the use of a dedicated servo system can thus increase the data storage capacity of a disc drive, it can also give rise to a limitation on the storage capacity. As will be clear to those of skill in the art, the data storage capacity of a disc drive depends upon the spacing of data tracks on the disc surfaces; the more closely the tracks are spaced, the larger the number of tracks that can be made available for the storage of data. However, as the track spacing is reduced, misalignments between the servo transducer and the data transducers and between the servo tracks and data tracks that would be negligible for a larger track spacing can result in sufficient misalignment between a data transducer and data tracks on the data surface proximate the data transducer that a file might be written to or read from the wrong data track if writing or reading takes place while the servo transducer is following a servo track. In the absence of some means for compensating for such misalignments, which can vary with the temperature of the disc stack and actuator, the density of tracks would have to be limited to a value such that the track spacing would be large in relation to any misalignments in the transducers and tracks that might occur.
Since limitation on the track density will unduly limit the data storage capacity of a disc drive, it is common practice to provide disc drives with a capability for following servo tracks with an offset that can be employed to compensate for the misalignments. To this end, a thermal calibration track is written to each data surface at the time of manufacture of the disc drive and the drive is provided with a thermal calibration circuit that can sample the thermal calibration tracks while the servo transducer is following the servo track in the same cylinder as the thermal calibration tracks to determine servo transducer offsets, for each data transducer, that will result in data track following by the data transducer. The offsets are expressed by a thermal calibration relation from which the appropriate amount of offset of the servo transducer can be determined for any orientation of the disc stack as measured from an index generated from the servo pattern. In the operation of the disc drive, the thermal calibration relation is determined on drive start up and, in many cases, from time to time thereafter, and subsequently used to align selected data transducers with data tracks on selected disc surfaces each time a file is stored or retrieved.
While thermal calibration has proved to be an effective method for increasing disc drive storage capacity, it has been found that a further limitation on track density occurs in disc drives that employ a dedicated servo surface. As will be clear to those of skill in the art, components of the thermal calibration circuit are subject to electronic offsets, that is, amplifiers used in the thermal calibration circuit generally will not have a zero output for a zero input. As a practical matter, these electronic offsets, per se, will not interfere with the thermal calibration of the disc drive. The electronic offsets, per se, will give rise to a systematic difference between the radii of the servo and data tracks but will not interfere with the use of the dedicated servo surface approach to track following. The electronic offsets cause a problem because they are not constant. Hence, after a disc drive has been formatted, changes in the electronic offsets will lead to mechanical offsets between the data tracks and data transducers in the same manner that mechanical offsets between the data and servo transducers will lead to offsets between the data tracks and data transducers. Moreover, since the problem exists in the thermal calibration circuit, it cannot be detected and compensated by measuring the thermal calibration relation using such circuit. Thus, in the past, it has been necessary to limit the track density to a value such that variations in electronic offsets of components of the thermal calibration circuit will have only a negligible effect on the thermal calibration relation that is determined using the thermal calibration circuit.